This invention relates to a deproteinizing agent for natural rubber latex which is used for producing a rubber product derived from natural rubber latex and decreased in content of allergy inducer to a level sufficient to keep it from being harmful to the human body, such as a surgical glove, a catheter, a condom, a foam rubber product or the like, as well as a method for producing a formed product of deproteinized natural rubber latex suitable for production of the above-described rubber product derived from natural rubber latex.
Natural rubber latex is commonly used as a material for a foam product such as a foam rubber product, a dipped product such as a glove, a condom or a catheter, a pressure-sensitive adhesive, an adhesive, and the like on an industrial scale. The natural rubber latex is obtained in the form of sap of a gum tree such as hevea brasiliensis or the like cultivated in a plantation and contains a rubber content in an amount of about 30%, as well as a non-rubber content including protein, fatty acids, polysaccharides, minerals and the like. The non-rubber content is contained in an amount of several percent. The natural rubber latex is called field latex. In order to permit the field latex to be used for an industrial material, it is required to be purified. For this purpose, the field latex is purified while being condensed to a degree sufficient to have a rubber content to be as high as about 60% and ammonia is added to the thus condensed and purified latex in an mount of 0.2 to 0.7% based on the latex in order to prevent putrefaction or rotting of the latex.
Such condensation and purification of the field latex may be carried out, for example, by creaming or centrifuging. In general, centrifuging is predominantly used to this end because it accomplishes purification of the field latex with increased efficiency. The centrifugal procedure permits purified natural rubber latex which is decreased in protein content to a level as low as about 2% to 3% by weight to be provided. About half of protein left in the purified natural rubber latex is water-extractable, to thereby act as a protective colloid in the latex, resulting in contributing to stabilization of the latex. The remaining protein is chemically bonded to rubber particles, to thereby be rendered hydrophobic. The hydrophobic protein bonded to the rubber particles permits the rubber particles to be stabilized in water and is readily oxidized to prevent oxidation and deterioration of the rubber.
In recent years, it is reported that use of medical supplies made of natural rubber such as a surgical glove and the like causes immediate hypersensitivity, to thereby give rise to a problem. The immediate hypersensitivity includes Type IV allergy represented by urticaria and Type I allergy causing dyspnea or anaphylaxis. It is confirmed that Type IV allergy is induced by a vulcanization accelerator blended in unvulcanized natural rubber and Type I allergy is induced by protein contained in the rubber.
Such allergies are caused when a patient having an antibody produced in the body due to contact with an allergy inducing material (hereinafter referred to as "allergen") such as protein or the like contained in natural rubber contacts with the allergen again. Thus, there would be likelihood that many latent patients are found in people commonly using a natural rubber product containing the allergen. For example, it is reported that an incident rate at which the medical interest commonly using a surgical glove or an examination glove made of natural rubber takes an allergic disease is increased to a level as high as 10%. The Food and Drug Administration (FDA) appeals manufacturers of such a natural rubber product to reduce a content of protein in a natural rubber material to be used. Thus, it will be noted that generation of allergies due to use of a natural rubber product gives rise to a serious social problem.
Although it is thus indicated that natural rubber gives rise to the above-described problem, natural rubber is significantly advantageous in that it is decreased in cost, exhibits increased toughness and permits a product made of natural rubber to exhibit satisfactory workability and fittingness. Unfortunately, there has not been found any substitute for natural rubber exhibiting such excellent properties. Thus, it is highly desirable to develop natural rubber which is decreased in allergen content to a level sufficient to keep it form being harmful to the human body.
Conventionally, a decrease in content of protein, which is a major allergen, in natural rubber is carried out by cleaning natural rubber latex or a natural rubber product with hot water or dipping it in a cleaning tank for a suitable length of time. Alternatively, it is carried out by subjecting a natural rubber product to a surface treatment using chemicals such as chlorine or the like. Unfortunately, such a procedure fails to remove the allergen from natural rubber to a degree sufficient to significantly reduce generation of allergies.
Also, techniques of providing deproteinized natural rubber latex using protease and a surfactant are proposed as disclosed in Japanese Patent Application Laid-Open Publications Nos. 56902/1994 (6-56902), 56903/1994 (6-56903), 56904/1994 (6-56904), 56905/1994 (6-56905) and 56906/1994 (6-56906).
The techniques disclosed accomplish removal of protein from natural rubber to a certain degree, however, it encounters another problem due to a surfactant used for removal of protein. The surfactant exhibits an important function for stabilization of the latex and cleaning of the protein. However, when it is left in an excess amount in natural rubber during a forming step, it deteriorates film forming properties of the rubber and decreases strength of a formed product. In particular, it is known in the art that a deterioration in film forming properties of the rubber appears in straight dip forming practiced for formation of a thin film such as a condom or the like or when an anionic surfactant is used as the surfactant. Also, when the surfactant is left in a formed product of natural rubber, it per se deteriorates safety of the product.
Further, the techniques disclosed require both operation for diluting natural rubber with water and operation for concentrating it by centrifuging or the like, to thereby cause an increase in the number of steps, leading to a decrease in yields of deproteinized natural rubber latex and a deterioration in quality thereof.